The present invention relates to the hardfacing of downhole tools with hard-metal powders and, more particularly, to plasma transferred arc torch systems for the internal injection of coarse hard-metal particles for hardfacing downhole tools.
“Hardfacing,” as that term is used herein, may be generally defined as a layer of hard, abrasion resistant material (referred to herein as a “deposit”) applied to a less resistant surface or substrate by a deposition technique. Hardfacing is frequently used to extend the service life of drill bits and other downhole tools used in the oil and gas industry. Tungsten carbide and various alloys of tungsten carbide are examples of hardfacing materials widely used to protect drill bits and other downhole tools associated with drilling and producing oil and gas wells. In effect, the carbide particles are suspended in a matrix of metal forming a layer on the surface of the tool. The carbide particles give the hardfacing material hardness and wear resistance, while the matrix metal provides fracture toughness to the hardfacing. The use of this technique has increased significantly over the years as industry has come to recognize that substrates of softer, lower, cost material can be hardfaced to have the same wear and corrosion resistance characteristics as more expensive substrates of a harder material.
Coating/hardfacing procedures consist of producing relatively thick deposits on substrates in order to give the latter the qualities inherent in the deposited material. Hardfacing involves the deposition of a hard layer by welding or thermal spraying. Conventional weld hardfacing is accomplished by oxyfuel welding (“OFW”), gas tungsten arc welding (“TIG”), gas metal arc welding (“GMAW”), shielded metal arc welding (“SMAW”), submerged arc welding (“SAW”), and flux-cored arc welding (“FCAW”). Plasma transferred arc hardfacing and laser beam hardfacing can also be used. Typically, a hardfacing material is applied, such as by arc or gas welding, to the exterior surface of a downhole tool, such as a drill bit, to protect the bit against erosion and abrasion. The hardfacing material typically includes one or more metal carbides, which are bonded to the steel body by a metal alloy (“binder alloy”).
Many factors affect the durability of a hardfacing composition in a particular application. These factors include the chemical composition and physical structure (size, shape, and particle size distribution) of the carbide particles, the chemical composition and microstructure of the matrix metal or alloy, and the relative proportions of the carbide materials to one another and to the matrix metal or alloy. The metal carbide most commonly used in hardfacing is tungsten carbide. Small amounts of tantalum carbide and titanium carbide may also be present in such material, although these other carbides may be considered to be deleterious.
Regardless of the type of hardfacing material used, designers continue to seek improved properties (such as improved wear resistance, thermal resistance, etc.) in the hardfacing materials. Unfortunately, increasing wear resistance usually results in a loss in fracture toughness, or vice-versa. For example, to achieve higher wear resistance (mainly against abrasion or erosion), the hardfacing composition may be designed to have a maximum amount of carbide content in the metallic matrix or the thickness of the hardfacing layer may be increased. However, a hardfacing with higher hardness and higher carbide content is more prone to cracking and delamination, especially as the thickness of the hardfacing increases. Furthermore, the tenacity or fracture toughness of a hardfacing layer decreases with an increased thickness of the single hardfacing layer, limiting the life of the hardfacing. One way to achieve a balance in abrasion or erosion resistance and toughness is to include larger carbide particles in the hardfacing composition. Larger particles have a larger surface area in the weld deposit that provides for enhanced mechanical properties.
When use is made of the plasma transferred arc (PTA) method, the transferred arc constitutes the heating element of the recharging material and the surface of the part forming the substrate. Referring to FIG. 1, a cross-sectional illustration of a PTA torch nozzle 116, a PTA torch nozzle has cathode holding device 102, plasma gas 104, cathode 106, cooling water 108, shielding gas 110, feedline 112 for the carbide powder from carbide powder hopper 120, weld deposit 114, substrate 118 (e.g., portion of a downhole tool that is being hardfaced), and power supply 122.
In a typical PTA device such as that shown in FIG. 1, the substrate is raised to a positive potential compared with the cathode of the torch and the plasma jet is then entirely traversed by electric current between the torch and the substrate, transmitting to the latter the energy necessary for heating and localized melting of the zone to be coated or hardfaced. The carbide powder is melted on the surface of the part in order to form a liquid weld deposit, which is continuously renewed during the displacement of the part beneath the torch. The surface melting of the substrate permits a metallurgical bond similar to that encountered in welding processes.
Although PTA methods are very useful in hardfacing techniques, the carbide powders that can be employed in those methods are limited in terms of particle sizes that can be used within the PTA nozzles. Normal torches feed fine carbide powders, which when placed on the substrate, are not as wear resistant and erosion resistant. The typical particle sizes that can be accommodated by PTA nozzles are about 250 microns to about 38 microns (60 mesh or finer). This is due to the clogging problems that results in the feedlines and in the nozzle of the PTA if larger particles are utilized in the carbide powder. Thus, heretofore, to utilize larger grain sizes of carbide particles, it is necessary to apply the carbide powder to the substrate with a manual welding technique utilizing carbide ropes or rods.
Manual application of the carbide powder can result in several issues due to human error and the inherent variability in a manual process. In terms of variability, the thickness of the weld deposit can vary greatly when applied manually. This may lead to additional techniques such as grinding with diamond grinding wheels of the coating to provide the desired thickness. Additionally, in a manual welding process, the substrate has to be pre-heated (e.g., to about 700° F., which requires additional time and exposes operators to high heats. Manual processes are also slower (e.g., at least three times) than automatic/robotic processes, which can lead to operator fatigue and accidents.